Among fuel cells, a solid oxide fuel cell (hereinafter referred to as “SOFC”) includes a solid electrolyte layer composed of a solid oxide and electrode layers disposed so as to sandwich the solid electrolyte layer.
In the SOFC, oxygen (air) is fed to an air electrode, and a fuel (H2, CH4, or the like) is fed to a fuel electrode. Each of the air electrode and the fuel electrode is porous so that the gas fed thereto can reach the interface with the solid electrolyte layer.
In the case where a hydrocarbon-containing gas such as natural gas (methane gas) is used as a fuel gas, it is necessary that the gas be reformed as a fuel gas containing hydrogen as a main component and that the reformed gas be allowed to act on the solid electrolyte. A steam reforming method is often used as a technique for reforming the fuel gas.
For example, when methane gas is used as the fuel gas, the methane gas is mixed with water vapor, and the mixture is heated to be decomposed into hydrogen and carbon monoxide, which is further reacted with water vapor to be decomposed into hydrogen and carbon dioxide. In many cases, a reformer which is used to perform reforming is installed outside a fuel cell, and the reformed fuel gas is fed to the fuel cell. When a reformer is installed outside a fuel cell, the scale of the apparatus increases and the apparatus becomes complicated, which is a problem. Accordingly, in some cases, an internal reforming method in which a fuel is reformed in a fuel electrode of a fuel cell may be used.
In the internal reforming method, part of the heat generated in the fuel cell can be used as a heat source for performing the steam reforming method and high-temperature waste heat can be recovered and used, and therefore, high energy efficiency is achieved. Accordingly, the internal reforming method has been receiving attention as a method for reforming fuel gas in SOFCs.